There is one theory that he (or they) worked for three letter agency because he picked one encryption used in bitcoin that doesn't have a backdoor.
You mean the secp256k1 elliptic curve? How do you know it doesn't have a backdoor?
To be honest, it doesn't take a government employee to know that the sec-2 curves did not have NIST's oversight on them like the P-123456 labeled curves. The NIST is known to just sit there and not challenge the NSA when it meddles with the algorithms,
like this one. So naturally there is more distrust for those elliptic curves than the sec-2 curves.
That being said, it is reasonable to believe that no sec-2 curve has an
NSA backdoor. Although that doesn't necessarily mean that they are mathematically hardened, as the safecurves website demonstrates.